net/http: ignore connection closes once done with the connection

Once the connection is put back into the idle pool, the request should
not take any action if the connection is closed.

Fixes #41600

Change-Id: I5e4ddcdc03cd44f5197ecfbe324638604961de84
Reviewed-on: https://go-review.googlesource.com/c/go/+/257818


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Damien Neil <dneil@google.com>